Do-while loop execution model in PHP - Step-by-Step Execution

Concept Flow - Do-while loop execution model

Start

↓

Execute body

↓

Check condition

Yes| No↓

Repeat

The do-while loop runs the code inside the loop first, then checks the condition. If true, it repeats; if false, it stops.

Execution Sample

PHP

<?php
$i = 1;
do {
  echo $i . " ";
  $i++;
} while ($i <= 3);
?>

This code prints numbers 1 to 3 using a do-while loop.

Execution Table

Step	Variable i	Condition ($i <= 3)	Action	Output
1	1	1 <= 3 is True	Print 1, increment i to 2	1
2	2	2 <= 3 is True	Print 2, increment i to 3	2
3	3	3 <= 3 is True	Print 3, increment i to 4	3
4	4	4 <= 3 is False	Exit loop

💡 i reaches 4, condition 4 <= 3 is False, loop stops

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
i	1	2	3	4	4

Key Moments - 2 Insights

Why does the loop body run even when the condition is false initially?

Because in a do-while loop, the body runs first before the condition is checked, as shown in step 1 of the execution_table.

When does the loop stop running?

The loop stops when the condition becomes false, which happens at step 4 when i is 4 and 4 <= 3 is false.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of i at step 3?

A 4

B 2

C 3

D 1

💡 Hint

Check the 'Variable i' column at step 3 in the execution_table.

At which step does the condition become false and the loop stops?

A Step 2

B Step 4

C Step 3

D Step 1

💡 Hint

Look at the 'Condition' column in the execution_table where it says false.

If the initial value of i was 5, how many times would the loop body execute?

A 1 time

B 0 times

C 3 times

D Infinite times

💡 Hint

Remember do-while runs the body once before checking condition, see concept_flow.

Concept Snapshot

do-while loop syntax in PHP:
do {
  // code to run
} while (condition);

Runs loop body first, then checks condition.
Repeats if condition true, stops if false.
Always runs at least once.

Full Transcript

A do-while loop in PHP runs the code inside the loop first, then checks the condition. If the condition is true, it repeats the loop. If false, it stops. This means the loop body always runs at least once, even if the condition is false initially. In the example, the variable i starts at 1. The loop prints i, then increases it by 1. It repeats while i is less than or equal to 3. When i becomes 4, the condition fails and the loop stops. This is shown step-by-step in the execution table and variable tracker.

Practice

1. What is the main difference between a do-while loop and a while loop in PHP?

easy

A. while loop always executes the code block at least once before checking the condition.

B. do-while loop executes the code block at least once before checking the condition.

C. do-while loop never checks the condition.

D. while loop executes the code block only if the condition is false.

Solution

1. Step 1: Understand the execution order of do-while loop

   A do-while loop runs the code block first, then checks the condition to decide if it should repeat.

2. Step 2: Compare with while loop behavior

   A while loop checks the condition first before running the code block, so it may not run at all if the condition is false initially.

3. Final Answer:

   do-while loop executes the code block at least once before checking the condition. -> Option B

4. Quick Check:

   do-while runs code once before condition check [OK]

Hint: Remember: do-while runs code first, then checks condition [OK]

Common Mistakes:

• Thinking while loop runs code before condition check
• Believing do-while skips condition check
• Confusing execution order of loops

2. Which of the following is the correct syntax for a do-while loop in PHP?

easy

A. do { /* code */ } while (condition);

B. do (condition) { /* code */ };

C. while (condition) { /* code */ } do;

D. do { /* code */ } until (condition);

Solution

1. Step 1: Recall PHP do-while syntax

   The correct syntax is: do { /* code */ } while (condition); with a semicolon after the while condition.

2. Step 2: Identify incorrect options

   Options A, C, and D use wrong keywords or misplaced parentheses, which are invalid in PHP.

3. Final Answer:

   do { /* code */ } while (condition); -> Option A

4. Quick Check:

   Correct do-while syntax ends with semicolon [OK]

Hint: Syntax: do { } while(condition); ends with semicolon [OK]

Common Mistakes:

• Omitting semicolon after while condition
• Using 'until' instead of 'while'
• Placing condition before code block

3. What will be the output of the following PHP code?

$i = 3;
do {
    echo $i . ' ';
    $i--;
} while ($i > 0);

medium

A. 3 2 1

B. 2 1 0

C. 3 2 1 0

D. 1 2 3

Solution

1. Step 1: Trace the loop iterations

   Initial value: $i = 3. Loop prints $i then decrements it. Iterations:
   1) print 3, $i=2
   2) print 2, $i=1
   3) print 1, $i=0

2. Step 2: Check loop condition after each iteration

   Loop continues while $i > 0. After printing 1, $i becomes 0, condition fails, loop stops.

3. Final Answer:

   3 2 1 -> Option A

4. Quick Check:

   Loop prints 3,2,1 then stops [OK]

Hint: do-while runs once before condition; count down from 3 [OK]

Common Mistakes:

• Including 0 in output
• Starting output from 1 instead of 3
• Confusing pre- and post-decrement

4. Identify the error in this PHP do-while loop:

$count = 1;
do {
    echo $count;
    $count++
} while ($count <= 3);

medium

A. Using do instead of while

B. Incorrect loop condition syntax

C. Missing semicolon after $count++

D. Variable $count not initialized

Solution

1. Step 1: Check each line for syntax errors

   The line $count++ is missing a semicolon at the end, which is required in PHP.

2. Step 2: Verify other parts of the loop

   The loop condition and variable initialization are correct. The use of do is proper for a do-while loop.

3. Final Answer:

   Missing semicolon after $count++ -> Option C

4. Quick Check:

   Every statement must end with semicolon [OK]

Hint: Check for missing semicolons inside loop [OK]

Common Mistakes:

• Forgetting semicolon after statements
• Misplacing loop condition parentheses
• Confusing do-while with while syntax

5. You want to read user input at least once and continue reading while the input is not empty. Which PHP do-while loop correctly implements this?

do {
    $input = trim(fgets(STDIN));
    // process input
} ???;

hard

A. while ($input !== '');

B. while ($input);

C. while ($input == '');

D. while (!empty($input));

Solution

1. Step 1: Understand the input condition

   We want to continue reading while input is not empty. The PHP function empty() returns true if input is empty.

2. Step 2: Choose the correct condition

   Using while (!empty($input)); means loop continues while input has some content, stopping when empty.

3. Final Answer:

   while (!empty($input)); -> Option D

4. Quick Check:

   Loop continues while input is not empty [OK]

Hint: Use while(!empty($input)) to loop until input is empty [OK]

Common Mistakes:

• Using equality to empty string incorrectly
• Using condition that stops loop too early
• Confusing empty() with isset()